Cure-On-Demand Liquid Sealant Composition, Process For The Preparation Thereof And Uses Thereof

ABSTRACT

The present invention is directed to a liquid sealant composition based on polysulphide, capable of curing on demand, and the process for preparing same, the liquid sealant composition including: (i) a polysulphide comprising an —SH end group, (ii) a crosslinking agent chosen from ethylenically unsaturated or acetylenically unsaturated compounds, the unsaturated ethylenic or unsaturated acetylenic functions of the crosslinking agent being chemically blocked with a blocking agent, and (iii) optionally, a catalyst. The present invention also relates to a process for coating a substrate with a composition according to the invention, and also to the cured sealant materials formed from this composition. Finally, the invention relates to the use of a liquid sealant composition according to the invention for the construction and/or the maintenance, and more particularly for the adhesive bonding and/or the protection, of vehicles and machines.

FIELD

The subject of the invention is a new liquid sealant composition basedon polysulphide, capable of curing on demand, and the process forpreparing same. The present invention also relates to a process forcoating a substrate with a composition according to the invention. Thecured sealant materials formed from this composition also form part ofthe invention. Finally, the invention also relates to the use of aliquid sealant composition according to the invention for theconstruction and/or the maintenance, and more particularly for theadhesive bonding and/or the protection, of vehicles and machines.

BACKGROUND

Polysulphide compositions in liquid and curable form have been used fora long time in the sealant field, owing to the durability andimpermeability of these polymers (ALIPS—Aliphatic Polysulfides—Monographof an elastomer—Heinz LUCKE, Hüthig & Wepf, 1994).

These sealant materials, commonly referred to as “mastics”, are used inparticular for the isolation, adhesive bonding or protection againstcorrosion of construction materials used in the construction, transportor aeronautical fields. They are generally two-component systemsconsisting of a base (or matrix) and a curing agent (or crosslinkingagent). It is during a mixing step that these two components reacttogether and crosslink so as to form the final mastic.

EP 0 619 355 describes a polysulphide system crosslinked with(meth)acrylates, using a guanidine derivative as catalyst. The reactionbrought about is very rapid (very short gel time), which leaves littletime for using the mastic. The gel times vary from less than one minuteto 20 minutes without retarder, and can be increased at most up to 7hours through the use of an acid retarder. These systems are thereforeneither perfectly controlled nor adjustable as desired, and the safetyperiod for use is not therefore “infinite”.

There are at the current time two routes for crosslinking polysulphide(PS) polymers comprising —SH end groups:

-   -   by oxidoreduction reaction: these are polysulphide/manganese        oxide (PS/MnO₂) systems which exhibit rather slow crosslinking        kinetics (hardening of the mastic), which can extend over        several days,    -   by addition reaction: these are polysulphide/epoxy resin        (PS/epoxy) systems which exhibit rather fast crosslinking        kinetics, but the reaction of which is incomplete (not total) at        ambient temperature. These systems have a high degree of        extraction which results in stiffening of the mastic when it is        brought to temperature.

The main drawback of these mastics is the need for a compromise betweena gel time sufficiently long for the safety period for use during theapplication, and crosslinking kinetics which are as rapid as possiblewith regard to the mastic becoming hard and to the handling thereof.

Thus, the concept of controlled-initiation Sealant Cure On Demand (SCOD)mastics has come to light. These mastics have a use and application timewhich is as long as possible and rapid setting kinetics once thecrosslinking has been initiated.

Patent EP 1 478 703 describes a process for coating a substrate byapplying a sealant material capable of curing on demand. The materialused can be based on polysulphide with an —SH end group, together with acrosslinking agent such as an acrylate, an isocyanate or an epoxy resin,said not yet cured material containing a latent catalyst which is formedand/or released in active form under the action of external energy, suchas heating or electromagnetic radiation, and by which the reactionbetween the polysulphide and the curing agent is initiated and/oraccelerated. This reaction results in the curing of the sealantmaterial. The latent catalyst used is an amine encapsulated in a polymershell. However, it is observed that this system does not enable completeblocking of the reaction between the polysulphide and the crosslinkingagent. A latency of the reaction is observed, which means that thereaction occurs, but is greatly slowed down. The latent catalyst retainsits primary function, namely that it is an accelerator of the reaction;its role is not to prevent the reaction between the polysulphide and thecuring agent. Thus, and since the reaction between the polysulphide andthe crosslinking agent is not totally blocked, the SCOD systems ofpatent EP 1 478 703 are necessarily “two-component” systems. Inaddition, the use of a latent catalyst in the form of capsules has manydisadvantages, namely that:

-   -   the latent catalyst capsules can be destroyed during the mixing        step, and result in an early loss of stability of the mastic,    -   the crosslinking reaction is heterogeneous owing to a        concentration effect of the latent catalyst, the dispersion of        the capsules is more difficult than in the case of a liquid        catalyst, which results in a deterioration of the final        properties of the mastic, and more particularly of the        mechanical properties and of the adhesion,    -   the stability of the catalyst capsules is limited over time        owing to diffusion phenomena,    -   the size of the capsules, even minimized, is not suitable for        certain applications such as, for example, interposition between        sheet metal, the thickness of the mastic then having to be very        thin, about from 100 to 200 μm.

SUMMARY

The present invention aims to overcome these drawbacks by developingliquid sealant compositions in which the reaction between the matrix andthe curing agent would be effectively blocked by making the reactivefunction of the curing agent inaccessible, independently of the presenceor absence of a catalyst. This concept allows the preparation of a“one-component” mastic system which is stable over time at ambienttemperature, and consequently stable during storage. This system can beirreversibly and rapidly cured on demand by virtue of the provision ofexternal (thermal, photochemical, etc.) energy. As a result, thestrategy of blocking the crosslinking according to the invention, bychemical blocking of the reactive function of the crosslinking agent,results in a more effective blocking of the mastics of the inventioncompared with the mastics of patent EP 1 478 703.

The liquid sealant compositions of the invention also allow the use ofcuring agents which are very reactive with respect to the polysulphide,without the risk of impairing the stability of the one-component system.

After curing, the compositions of the invention result in crosslinkedmastics exhibiting excellent mechanical and chemical properties:

-   -   a tensile strength of at least 1 MPa,    -   an elongation at break of at least 100%,    -   good low-temperature (˜−53° C.) resistance, and    -   good chemical resistance to jet fuel.

For all these reasons, the compositions of the invention correspond muchmore satisfactorily to the SCOD concept as it is really defined, thanthe solutions hitherto described in the prior art (a scheme of theprinciple of an ideal SCOD system is represented in FIG. 1).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a scheme of the principle of an ideal SCOD system.

DETAILED DESCRIPTION

Thus, the first subject of the invention is a liquid sealant compositioncomprising at least:

(i) a polysulphide with an —SH end group,

(ii) a crosslinking agent chosen from ethylenically unsaturated oracetylenically unsaturated compounds, said unsaturated ethylenic orunsaturated acetylenic functions being chemically blocked with ablocking agent, by Diels-Alder reaction,

(iii) optionally, a catalyst.

The blocking agent serves to protect the unsaturated ethylenic orunsaturated acetylenic function of the crosslinking agent. It issubsequently removed under the effect of external energy which causesthe release of the unsaturated ethylenic or unsaturated acetylenicfunction, and which results in the initiation of the reaction betweenthe polysulphide and the crosslinking agent and therefore in the curingof the mass.

The polysulphide of the invention preferably has a —SH functionality ≧2.Said polysulphide can have an average molecular weight of between 1000and 6500 g·mol⁻¹, and preferably between 2000 and 3500 g·mol⁻¹. Itsviscosity can be between 1 and 150 Pa·s, and preferably between 10 and50 Pa·s. The Tg of the polysulphide can be between −55 and −65° C.

Advantageously, the crosslinking agent of the invention has afunctionality f≧2, and preferably 2≦f≦4. The crosslinking agent can be acompound or a mixture of several compounds bearing an allyl function, amaleimide function, or an ethylenically unsaturated or acetylenicallyunsaturated function bearing an electron-withdrawing group in the alphaposition with respect to this double bond or this triple bond, it beingpossible for said function to be an aldehyde function, ketone function,ester function, hemiester function, amide function, hemiamide function,ester-amide function, nitrile function or imide function which isα,β-ethylenically unsaturated. The crosslinking agents which are mostpreferred are maleimides, hemiesters and ester-amides.

The stoichiometric ratio between the ethylenic function of the curingagent and the —SH group of the polysulphide can range from 1 to 1.4.

According to one particularly advantageous embodiment, the crosslinkingagent is an α,β-ethylenically unsaturated imide compound or a mixture ofseveral α,β-ethylenically unsaturated imide compounds chosen frommonomaleimide, bismaleimide or trismaleimide compounds of formula:

in which:

-   -   R is chosen from linear or branched C₁-C₃₀ alkyl chains,    -   R_(a), R_(b), R_(c), R_(d), R_(e) and R_(f), which may be        identical or different, are chosen from H or linear or branched        C₁-C₃₀ alkyl chains, and    -   W is a polyether, polythioether, polyester, polyamide or        polysulphide unit.

By definition, a polysulphide is a polymer comprising at least one S—Sbond in its chain.

The W unit may be a C₁-C₃₀ alkyl chain comprising one or moreheteroatoms chosen from O, S or N.

In the case where W is a polyether unit, it is preferably a polyethyleneglycol (PEG) unit, a polypropylene glycol (PPG) unit, or a mixed PEG/PPGunit. The crosslinking agent is then preferably chosen from thecompounds of formula:

in which:

-   -   R_(a), R_(b), R_(c), R_(d), R_(e) and R_(f), which may be        identical or different, are chosen from H or linear or branched        C₁-C₃₀ alkyl chains,    -   A and B, which may be identical or different, are linear or        branched C₁-C₃₀ alkyl chains which can comprise, in their chain,        one or more heteroatoms chosen from O, S or N,    -   Y is chosen from —CH₂—, —CH₂—CH₂—, —CH₂—CH₂—CH₂—, —CH₂—CH(CH₃)—        and —CH₂—CH₂—CH₂—CH₂,    -   x, x′, x″, y, y′, y″, z, z′ and z″, which may be identical or        different, range from 0 to 70, preferably from 1 to 40, and even        more preferentially from 1 to 12,    -   the sums x+y+z, x′+y′+z′ and x″+y″+z″ are strictly greater than        1, and preferably between 1 and 15,    -   n ranges from 0 to 6, and preferably n=0 or 1,    -   R′ is H or a C₁-C₆ alkyl radical, and preferably R′ is H, CH₃ or        C₂H₅,        and in which the ethylenic bond of the monomaleimide ring or the        ethylenic bonds of the bismaleimide or trismaleimide rings are        blocked by Diels-Alder reaction with said blocking agent.

In the case where W is a polythioether or polysulphide unit, said unitmay be a C₁-C₃₀ alkyl chain comprising one or more O heteroatoms or oneor more S heteroatoms. It is preferably a unit of formula:

—CH₂—CH₂—CH₂—S—CH₂—CH₂—O—CH₂—CH₂—S—CH₂—CH₂—CH₂—, or

—CH₂—CH₂—CH₂—S—[CH₂—CH₂—O—CH₂—O—CH₂—CH₂—S—S]_(n′)—CH₂—CH₂—O—CH₂—O—CH₂—CH₂—S—CH₂—CH₂—CH₂—,inwhich n′ ranges from 0 to 7.

The blocking agent may be a diene comprising at least one pentacyclicnucleus, and preferably a diene corresponding to the following formula:

in which:

-   -   R₁, R₂, R₃ and R₄, which may be identical or different, are        chosen from H or C₁-C₁₂ alkyl chains optionally functionalized        with at least one ester, ether, ketone or amide function,    -   X═CH₂, O, S, or N—R_(x) in which R_(x) is chosen from linear or        branched C₁-C₃₀ alkyl chains.

According to one advantageous embodiment, X═O.

According to another advantageous embodiment:

-   -   the R₁ radical and optionally the R₄ radical are identical        C₁-C₁₂ alkyl chains optionally functionalized with at least one        ester, ether, ketone or amide function, and preferably chosen        from the following groups:

—R_(y)—C(O)O—R_(z),—R_(y)—(O)CO—R_(z),and —R_(y)—O—R_(z)

in which:

-   -   R_(y) is a linear or branched C₁-C₈, preferably C₁-C₄, and even        more preferentially C₁-C₂, alkyl chain,    -   R_(z) is a linear, branched or cyclic C₁-C₁₂, preferably C₁-C₆,        alkyl chain, and even more preferentially a linear C₁-C₂ alkyl        chain, or an aryl group, and preferably a phenyl group,        -   the R₂ radical, the R₃ radical and, optionally, the R₄            radical, if it is not a C₁-C₁₂ alkyl chain as mentioned            above, are hydrogen atoms.

The blocking agents which are the most preferred are chosen from:

-   -   furfuryl acetate of formula:

-   -   furfuryl benzyl of formula:

-   -   ethyl 3-(2-furyl)propanoate of formula:

-   -   furan-2-ylmethyl pivalate of formula:

-   -   furan-2-ylmethyl-3,5,5-trimethyl hexanoate of formula:

-   -   furan-2-ylmethylcyclohexanecarboxylate of formula:

The catalyst used is preferably chosen from:

-   -   tertiary amines, such as 1,4-diazabicyclo[2,2,2]octane (DABCO or        TEDA), 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU),        1,5-diazabicyclo[4,3,0]non-5-ene (DBN),        N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA),        N-methyl-N′-(dimethylaminoethyl)piperazine,        N,N-dimethylcyclohexylamine (DMCHA),        N,N′,N″-tris(dimethylaminopropyl)hexahydrotriazine or        bis(2-(dimethylamino)ethyl)ether (BDMAEE),    -   guanidine and derivatives thereof, such as diphenylguanidine,        tetramethylguanidine or di-o-tolylguanidine,    -   morpholine and derivatives thereof, such as N-methylmorpholine        (NMM), N-ethylmorpholine (NEM), dimorpholinodiethyl ether        (DMDEE), or N-methylmorpholine oxide (NMMO),    -   Lewis acids, such as FeCl₃, AlCl₃ or SnCl₂, and    -   tin salts, such as dibutyl tin dilaurate (DBTDL) or dioctyl tin        dilaurate (DOTDL).

The catalysts which are the most preferred are tertiary amines, and moreparticularly 1,4-diazabicyclo[2,2,2]octane (DABCO or TEDA) and1,8-diazabicyclo[5,4,0]undec-7-ene (DBU).

The amount of catalyst used ranges from 0 to 10 parts by weight per 100parts of polysulphide polymer, and preferably from 0.5 to 5 parts.

Another subject of the invention relates to a process for preparing aliquid sealant composition according to the invention, namely a“ready-to-use” one-component mastic, comprising at least the followingsteps:

-   -   i) synthesis of the crosslinking agent as defined according to        the invention, chemically blocked by Diels-Alder reaction with a        blocking agent as previously defined, optionally in the presence        of a catalyst, then    -   ii) mixing of the chemically blocked crosslinking agent obtained        at the end of step i) with a liquid polysulphide polymer        comprising a —SH end group, optionally in the presence of a        catalyst, the mixing temperature having to be controlled so as        not to exceed the crosslinking agent deblocking temperature        (cooling optionally necessary), this temperature having to be        below 65° C., and preferably between 20 and 40° C.

The mixing step ii) can be carried out mechanically using a mixer, forinstance a Dispermat® blade (butterfly or notched) mixer, or a planetarymixer (mixer comprising a blade and one or more dispersers).

The “ready-to-use” liquid sealant composition or mastic thus prepared issubsequently packaged in a pot or in a cartridge.

The invention also relates to a coating process for attaching oradhesively bonding sealing trims and/or pieces or filling hollow spacesor intermediate gaps, said process comprising a step of applying aliquid sealant composition according to the invention to a substrate(made of aluminium, steel, glass, composites, etc.), followed by a stepof deprotecting the crosslinking agent chemically blocked with ablocking agent, under the effect of external energy, which results inthe initiation of the reaction between the polysulphide and thecrosslinking agent, and therefore in the curing of the mass of thesealant material.

The external energy required for deblocking the crosslinking agent canbe thermal energy provided:

-   -   either by conduction: heating by direct contact, by the Joule        effect via an electrical resistance, or by induction,    -   or by convection: provision of hot air, for example using a        Leister® hot air gun,    -   or by radiation: by infrared lamp, by high-frequency pulses such        as microwaves, ultrahigh frequencies (UHF) or ultrasound.

According to one advantageous embodiment, the crosslinking agentchemically blocked with a blocking agent is deprotected by heating at atemperature in the range of from 60 to 120° C., and preferably from 70to 100° C. Preferably, the crosslinking agent chemically blocked with ablocking agent is subjected to the action of heat for a period rangingfrom 15 minutes to 2 hours.

The coating process of the invention may also comprise, after the stepof deprotecting the crosslinking agent, a step of curing the mass of thesealant material. This curing step is preferably carried out at atemperature ranging from 18 to 35° C., for a period ranging from 1 to 3days.

Another subject of the invention relates to a cured sealant materialwhich results from the curing of a liquid sealant composition accordingto the invention, and which exhibits, after complete curing, a tensilestrength according to standard ISO 37 of at least 1 MPa, a tensileelongation at break according to standard ISO 37 of at least 100%, and ahardness according to standard ISO 7619 of at least 40 shA.

Finally, a last subject of the present invention relates to the use of aliquid sealant composition according to the invention for constructingand/or maintaining aircraft or spacecraft, motor vehicles, railvehicles, ships, machines, appliances and furniture, and moreparticularly for adhesive bonding and/or protection against corrosion ofaircraft or spacecraft, motor vehicles, rail vehicles, ships, machines,appliances and furniture.

In addition to the preceding arrangements, the invention also comprisesother arrangements which will emerge from the following furtherdescription, which refers to examples demonstrating the advantageousproperties of the compositions of the invention, and also to theappended FIG. 1 which represents a scheme of the principle of an idealSCOD system.

EXPERIMENTAL SECTION Example 1

This example studies the preparation of a crosslinking agent, themaleimide function of which is blocked by Diels-Alder (DA) reaction, andalso the deblocking conditions, and more specifically the temperature ofthe retro-Diels-Alder (r-DA) reaction.

The study of the chemical blocking and deblocking of the maleimidefunction is carried out on a bifunctional model compound, with severalblocking agents, namely furan derivatives of different natures. Thedetermination of the DA and r-DA reaction temperatures is measured bymeans of NMR monitoring of the blocked maleimide, by gradual increase inthe temperature.

TABLE 1 DA/rDA temperature (° C.) Diastereoisomer 1 Diastereoisomer 2 DArDA DA rDA

40 75 40 95

40 65 40 95

40 75 40 95

40 75 40 95

40 65 40 95

40 75 40 95

Surprisingly, the furan derivatives make it possible to block themaleimide function at “low temperature”, the DA temperatures being 40°C. (it being possible for these temperatures to be brought back toambient temperature by increasing the reaction time from 4 hours to 4days). Likewise, the r-DA temperatures are between 65 and 95° C., thusmaking it possible to envisage deblocking at “moderate temperature”, andwhich is consequently compatible with the intended final application andthe thermal activation envisaged.

Example 2

This example studies the preparation of a polysulphide mastic bycrosslinking of a liquid polysulphide (PS) polymer with a bismaleimide.Two liquid bismaleimides having a PPG-type backbone are tested inparallel: one synthesized from PPG 230 (Jeffamine D230, Huntsman) andthe other from PPG 400 (Jeffamine D400, Huntsman). The reactivity ofthese systems is studied comparatively on the bismaleimides in theblocked and nonblocked state. In the case of the blocked bismaleimide,the deblocking of the bismaleimide crosslinking agent, and therefore theactivation of the crosslinking, is carried out by heating in an oven.

The blocked bismaleimide crosslinking agents are prepared beforehand.

In a first step, the liquid bismaleimides are synthesized, then purifiedand characterized by NMR.

In a second step, these bismaleimides are blocked by Diels-Alderreaction, via a furan-derived blocking agent, furfuryl acetate. NMRverification makes it possible to check that the double bonds of thebismaleimides are actually blocked.

The procedures implemented are detailed hereinafter.

Procedure for Synthesis of Bismaleimides Based on PPG Diamines:

Maleic anhydride (2.1 eq) is solubilized in THF (2 ml/g of anhydride),in a three-necked round-bottomed flask. The diamine (1 eq) solubilizedin tetrahydrofuran (THF) (2 ml/g of diamine) is added dropwise using adropping funnel. The mixture is left to react for 2 hours at ambienttemperature.

The apparatus is then placed under nitrogen for a period of 10 minutes,and then ZnBr₂ (2.3 eq) is added under a nitrogen stream. Once the solidis completely solubilized, the mixture is heated to 40° C., and then asolution of bis(trimethylsilyl)amine (HMDS) solubilized in THF (1 ml/mlof HMDS) is added dropwise using a dropping funnel. The mixture isheated at 70° C. for a period of 3 hours. A viscous brown liquid isobtained.

The viscous liquid is then extracted via a liquid/liquid extraction withwater and chloroform (1/3 ratio). The operation is carried out threetimes, in order to remove any trace of water due to the cyclization(2^(nd) step). After having dried the organic phase with anhydrousNa₂SO₄, the solvent is evaporated off. Finally, the product obtained issolubilized in toluene (3 ml/g of product), and then heated at 100° C.overnight, in order to obtain exclusively the maleimide.

Blocking of the Maleimides by Diels-Alder Reaction:

The bismaleimide to be blocked is placed in a single-neckedround-bottomed flask. Solvent (THF or DCM, 1 ml/g of product) can beadded if the product is too viscous. Once the product has dissolved, theblocking agent is added in stoichiometric amount, and then the mixtureis left to react for 3 to 4 days at ambient temperature, or 4 hours at40° C. Two diastereoisomers are obtained (endo and exo), demonstrated byNMR verification.

The furfuryl acetate-blocked PPG 230 bismaleimide is analysed by NMR:

δ_(H) (400 MHz, CDCl₃): 6.61-6.51 (d, 2H, H-8′), 6.43-6.31 (m, 2H, H-8),5.3 (d, 1H, H-7exo) and 5.25 (d, 1H, H-7endo), 4.87-4.57 (dd, 2H,H-9exo) and 4.97-4.37 (dd, 2H, H-9endo), 4.46-4.26 (s, 2H, H-1),3.96-3.17 (m, 10H, H-3, H-5, H-4), 2.99-2.82 (dm, 2H, H-6′ exo/endo,H-6exo/endo), 2.14 (s, 3H, H-10exo) and 2.11 (s, 3H, H-10endo),1.31-0.97 (m, 12H, H-2 and H-11).

The furfuryl acetate-blocked PPG 400 bismaleimide is analysed by NMR:

δ_(H) (400 MHz, CDCl₃): 6.54-6.52 (d, 2H, H-8′), 6.40-6.33 (m, 2H, H-8),5.27 (d, 1H, H-7exo) and 5.24 (s, 1H, H-7endo), 4.84-4.55 (dd, 2H,H-9exo) and 4.95-4.36 (dd, 2H, H-9endo), 4.37-4.27 (s, 2H, H-1),3.91-3.26 (m, 20H, H-3, H-5, H-4), 2.92-2.79 (dm, 2H, H-6′ exo/endo,H-6exo/endo), 2.10 (s, 3H, H-10exo) and 2.08 (s, 3H, H-10endo),1.29-1.01 (m, 22H, H-2 and H-11).

The liquid sealant compositions are subsequently prepared from:

-   -   a liquid polysulphide polymer, Thioplast® G21 (Akzo Nobel),    -   a crosslinking agent, incorporated into the PS in the nonblocked        state or the blocked state (blocked with furfuryl acetate),        chosen from the two previously synthesized bismaleimides, having        the following formula:

in which n″=2 (BM PPG 230) or n″=5-6 (BM PPG 400),

-   -   optionally a catalyst: 1,4-diazabicyclo[2,2,2]octane (DABCO        33-LV®, Air Products).

The amounts of polysulphide and of crosslinking agent to be mixedobserve the stoichiometry of 1.1 between the ethylenic functions of thebismaleimide and the thiol functions of the polysulphide. The catalyst,if it is used, is added in an amount of 3 parts (per hundred parts ofpolysulphide). All the constituents are mixed manually or mechanicallyusing a Dispermat® blade mixer/disperser.

The gel time, which characterizes the time for the mastic to start toset (start of formation of the three-dimensional crosslinking network),and therefore the end of the safety period for using the mastic, isdetermined by a simple bench test. The test is carried out in analuminium dish, on the basis of 20 g of polysulphide. The measurementbegins as soon as the polysulphide and the crosslinking agent arebrought into contact (start of the mixing step). The increase inviscosity of the system is evaluated qualitatively by stirring themixture with a spatula, until the “gel time” or “string time” isreached, i.e. the moment at which the system begins to become elasticand begins to form a “string” when trying to withdraw the spatula.

TABLE 2 Crosslinking agent Temperature used Catalyst (in ° C.) Gel timeBM/PPG 230 DABCO AT ~5 min — AT 2 h 20 BM/PPG 230 BLQ DABCO AT no gelformation after 11 days DABCO 100° C. 1 h 15 BM/PPG 400 DABCO AT 8 minBM/PPG 400 BLQ DABCO AT no gel formation after 7 days DABCO 100° C. 1 h35 AT = ambient temperature

The results of Table 2 show the strong reactivity of the bismaleimideswith the polysulphide at ambient temperature (short gel time, <10 min inthe presence of catalyst), and the very great stability of these systemswith the blocked bismaleimides. The effectiveness of chemical blockingof a very reactive polysulphide/bismaleimide system, in order to obtaina very long mastic application time (at least 7 days of stability), isthus observed. Furthermore, these “ready-to-use” one-component masticsare stable during storage.

Bringing these systems with the blocked bismaleimides to temperature(100° C. in an oven) clearly makes it possible to initiate the reaction(gel time of 1h15-1h35).

A trifunctional blocked maleimide compound corresponding to thefollowing formula was also prepared:

Procedure for Synthesis of the Trismaleimide Compound:

Maleic anhydride (3.1 eq) is solubilized in THF (2 ml/g of anhydride),in a three-necked round-bottomed flask. The Jeffamine® T-403 triaminefrom Huntsman (1 eq) solubilized in THF (2 ml/g of triamine) is thenadded dropwise using a dropping funnel. The mixture is left to react for2 hours at ambient temperature.

The apparatus is then placed under nitrogen for 10 minutes, and thenZnBr₂ (3.6 eq) is added under a nitrogen stream. Once the solid istotally solubilized, the mixture is heated to 40° C., then a solution ofHMDS (3.9 eq), solubilized in THF (1 ml/ml of HMDS) is added dropwiseusing a dropping funnel. Finally, the mixture is heated at 70° C. for 3hours. A viscous brown liquid is obtained.

The viscous liquid then undergoes a liquid/liquid extraction with waterand chloroform (1/3 respectively); the operation is repeated three timesin order to remove any trace of water due to the cyclization (2^(nd)step). After having dried the organic phase with anhydrous Na₂SO₄, thesolvent is evaporated off. The product obtained is then solubilized intoluene (3 ml/g of product), and then heated at 100° C. for two days, inorder to obtain exclusively the maleimide.

The blocking of this trismaleimide crosslinking agent is carried out byDiels-Alder reaction, according to the same procedure as that describedpreviously for the bismaleimides.

A mastic was then prepared from:

-   -   a liquid polysulphide polymer, Thioplast® G21 (Akzo Nobel),    -   a crosslinking agent, incorporated into the PS, composed of a        mixture of the bismaleimide (BM PPG 230) and of the        trismaleimide corresponding to the formula above, both blocked        with furfuryl acetate, in proportions such that the average        maleimide functionality is 2.6, and    -   optionally, a catalyst: 1,4-diazabicyclo[2,2,2]octane (DABCO        33-LV®, Air Products).

The mastic was subsequently characterized in a standard manner, after 14days at ambient temperature, once the temperature deblocking had beencarried out by placing in an oven at a temperature of 100° C. for aperiod of 1h30:

-   -   Breaking strength (ISO 37): 1.2 MPa;    -   Elongation at break (ISO 37): 143%;    -   Hardness (ISO 7619): 46 shA;    -   Tg (determined by DSC)=−51° C.;    -   Adhesion in simple shear (on an aluminium test piece,        adhesive-coated surface 25×25 mm, mastic thickness 200 μm): 1.2        MPa, surfaces of fracture 100% cohesive (fracture in the        material).

Counterexample:

The compositions evaluated in the counterexample are representative ofthe prior art described in patent EP 1 478 703. They are PS/epoxy resinor PS/acrylate reactive systems, with a “latent” catalyst, namely anencapsulated tertiary amine.

The PS/epoxy resin system is a Thioplast® G21 (Akzo Nobel)/Novolac DEN431 epoxy resin (Dow Chemical) system, functionality 2.8 and EEW(Epoxide Equivalent Weight)=175 g/mol.

The PS/acrylate system is a Thioplast® G21 (Akzo Nobel)/SR 454(Sartomer): triacrylate (TMPEOTA, ethoxylated trimethylolpropanetriacrylate) system.

The encapsulated amine catalyst was prepared in the form ofmicroparticles based:

-   -   on porous silica,    -   on tertiary amine: Polycat® SA2/LE (Air Products)—DBU retarded,        and    -   on plant wax (melting point ˜75° C.).

Firstly, the amine in liquid form is adsorbed onto the silica. Thesilica particles containing the amine are then coated with wax in afluidized airbed (vaporization of the wax in the molten state).

The formulae corresponding to the systems evaluated are given in Table 3below:

TABLE 3 PS/epoxy resin PS/acrylate Thioplast ® G 21 100 100 DEN 431 x SR454 x DBU catalyst (in encapsulated 1.5 1.5 or free form), active amountx: The amount of epoxy resin or of acrylate added to the polysulphidepolymer is calculated so as to observe the epoxy or acrylate/—SH = 1.1stoichiometric ratio.

In the two cases, the systems are prepared by manually mixing thepolysulphide, the crosslinking agent (epoxy resin or acrylate) and theamine catalyst (liquid in its free form or else encapsulated in itslatent form). The manual mixing allows “gentle” mixing of theconstituents, and thus avoids any damage to the catalyst microparticleswhich could generate early activation of the reactive system.

The two systems representative of patent EP 1 478 703 are compared witha system according to the invention: PS/BM PPG 230 (blocked ornonblocked) as described in Example 2.

The gel times (determined at 23° C.) of these various systems arereproduced in Table 4 below:

TABLE 4 Composition according Composition according to patent EP 1 478703 to the invention PS/epoxy resin PS/acrylate PS/maleimide Freecatalyst ~11 min <1 min Nonblocked ~5 min maleimide “Encapsulated”  6-7days 24 h Blocked >4 months catalyst maleimide No catalyst <45 days  7days

These results show the weak stability of the systems with a latentcatalyst (encapsulated amine) in comparison with the system withchemical blocking of the invention. The solution with chemical blockingalso makes it possible to improve the storage time, and makes itpossible to propose a truly one-component SCOD mastic.

1. Liquid sealant composition comprising at least: (i) a polysulphidewith an —SH end group, (ii) a crosslinking agent chosen fromethylenically unsaturated or acetylenically unsaturated compounds, saidunsaturated ethylenic or unsaturated acetylenic functions of thecrosslinking agent being chemically blocked with a blocking agent, (iii)optionally, a catalyst.
 2. Composition according to claim 1, whereinsaid unsaturated ethylenic or unsaturated acetylenic functions of thecrosslinking agent are chemically blocked by Diels-Alder reaction withsaid blocking agent.
 3. Composition according to claim 1 wherein saidcrosslinking agent has a functionality f≧2, and preferably 2≦f≦4. 4.Composition according to claim 1, wherein said crosslinking agent is acompound or a mixture of several compounds bearing an allyl function, amaleimide function, or an ethylenically unsaturated or acetylenicallyunsaturated function bearing an electron-withdrawing group in the alphaposition with respect to this double bond or this triple bond, it beingpossible for said function to be an aldehyde function, ketone function,ester function, hemiester function, amide function, hemiamide function,ester-amide function, nitrile function or imide function which isα,β-ethylenically unsaturated.
 5. Composition according to claim 4,wherein said crosslinking agent is an α,β-ethylenically unsaturatedimide compound or a mixture of several α,β-ethylenically unsaturatedimide compounds chosen from monomaleimide, bismaleimide or trismaleimidecompounds of formula:

in which: R is chosen from linear or branched C₁-C₃₀ alkyl chains,R_(a), R_(b), R_(c), R_(d), R_(e) and R_(f), which may be identical ordifferent, are chosen from H or linear or branched C₁-C₃₀ alkyl chains,and W is a polyether, polythioether, polyester, polyamide orpolysulphide unit.
 6. Composition according to claim 5, wherein saidcrosslinking agent is a monomaleimide, bismaleimide or trismaleimidecompound in which W is a polyethylene glycol (PEG) unit, a polypropyleneglycol (PPG) unit, or a mixed PEG/PPG unit, and is chosen from thecompounds of formula:

in which: R_(a), R_(b), R_(c), R_(d), R_(e) and R_(f), which may beidentical or different, are chosen from H or linear or branched C₁-C₃₀alkyl chains, A and B, which may be identical or different, are linearor branched C₁-C₃₀ alkyl chains, which can comprise, in their chain, oneor more heteroatoms chosen from O, S or N, Y is chosen from —CH₂—,—CH₂—CH₂—, —CH₂—CH₂—CH₂—, —CH₂—CH(CH₃)— and —CH₂—CH₂—CH₂—CH₂, x, x′, x″,y, y′, y″, z, z′ and z″, which may be identical or different, range from1 to 70, preferably from 1 to 40, and even more preferentially from 1 to12, the sums x+y+z, x′+y′+z′ and x″+y″+z″ are strictly greater than 1,and preferably between 1 and 15, n ranges from 0 to 6, and preferablyn=0 or 1, R′ is H or a C₁-C₆ alkyl radical, and preferably R′ is H, CH₃or C₂H₅, and in which the ethylenic bond of the monomaleimide ring orthe ethylenic bonds of the bismaleimide or trismaleimide rings areblocked by Diels-Alder reaction with said blocking agent.
 7. Compositionaccording to claim 1, wherein said blocking agent is a diene comprisingat least one pentacyclic nucleus.
 8. Composition according to claim 7,wherein said blocking agent is a diene corresponding to the followingformula:

in which: R₁, R₂, R₃ and R₄, which may be identical or different, arechosen from H or C₁-C₁₂ alkyl chains optionally functionalized with atleast one ester, ether, ketone or amide function, X═CH₂, O, S, orN—R_(x), in which R_(x) is chosen from linear or branched C₁-C₃₀ alkylchains.
 9. Composition according to claim 8, in which X═O. 10.Composition according to claim 8, in which: the R₁ radical andoptionally the R₄ radical are identical C₁-C₁₂ alkyl chains optionallyfunctionalized with at least one ester, ether, ketone or amide function,the R₂ radical, the R₃ radical and, optionally, the R₄ radical, if it isnot a C₁-C₁₂ alkyl chain, are hydrogen atoms.
 11. Composition accordingto claim 10, in which the R₁ radical and optionally the R₄ radical arechosen from the following groups:—R_(y)—C(O)O—R_(z),—R_(y)—(O)CO—R_(z),and R_(y)—O—R_(z), in which: R_(y)is a linear or branched C₁-C₈, preferably C₁-C₄, and even morepreferentially C₁-C₂, alkyl chain, R_(z) is a linear, branched or cyclicC₁-C₁₂, preferably C₁-C₆, alkyl chain, and even more preferentially alinear C₁-C₂ alkyl chain, or an aryl group, and preferably a phenylgroup.
 12. Composition according to claim 1, wherein said blocking agentis chosen from: furfuryl acetate, furfuryl benzyl, ethyl3-(2-furyl)propanoate, furan-2-ylmethyl pivalate,furan-2-ylmethyl-3,5,5-trimethyl hexanoate andfuran-2-ylmethylcyclohexanecarboxylate.
 13. Process for preparing aliquid sealant composition as defined according to claim 1, wherein theprocess comprises at least the following steps: i) synthesizing of acrosslinking agent chosen from ethylenically unsaturated oracetylenically unsaturated compounds, said unsaturated ethylenic orunsaturated acetylenic functions of the crosslinking agent beingchemically blocked by Diels-Alder reaction with a blocking agent,optionally in the presence of a catalyst, then ii) mixing of thechemically blocked crosslinking agent obtained at the end of step i)with a liquid polysulphide polymer comprising an —SH end group,optionally in the presence of a catalyst, the mixing temperature havingto be controlled so as not to exceed the crosslinking agent deblockingtemperature.
 14. Coating process comprising a step of applying acomposition as defined according to claim 1 to a substrate, followed bya step of deprotecting the crosslinking agent chemically blocked with ablocking agent, under the effect of external energy, which results inthe initiation of the reaction between the polysulphide and thecrosslinking agent, and therefore in the curing of the mass of thesealant material.
 15. Process according to claim 14, in which thecrosslinking agent chemically blocked with a blocking agent isdeprotected by retro-Diels-Alder.
 16. Process according to claim 14, inwhich the external energy is provided by conduction, by convection or byradiation.
 17. Process according to one of claim 14, in which thecrosslinking agent chemically blocked with a blocking agent isdeprotected by heating at a temperature in the range of from 60 to 120°C.
 18. Process according to claim 17, in which the crosslinking agentchemically blocked with a blocking agent is subjected to the action ofheat for a period ranging from 15 minutes to 2 hours.
 19. Processaccording to claim 14, in which, after the step of deprotecting thecrosslinking agent, the mass of sealant material is subjected to acuring step.
 20. Process according to claim 19, in which the curing stepis carried out at a temperature ranging from 18 to 35° C., for a periodranging from 1 to 3 days.
 21. Cured sealant material prepared from thecuring of a liquid sealant composition as defined according to claim 1and in that it exhibits, after complete curing, a tensile strength of atleast 1 MPa, a tensile elongation at break of at least 100%, and ahardness of at least 40 shA.
 22. Use of a liquid sealant composition asdefined according to claim 1 for constructing and/or maintainingaircraft or spacecraft, motor vehicles, rail vehicles, ships, machines,appliances and furniture.
 23. Use of a liquid sealant composition asdefined according to claim 22 for adhesive bonding and/or protectionagainst corrosion of aircraft or spacecraft, motor vehicles, railvehicles, ships, machines, appliances and furniture.